Tens of thousands of people massed Friday in Baghdad to pray and chant slogans in support of the rebellious cleric Moqtada Sadr. U.S. forces, meanwhile, continued attacks in two Iraqi cities that insurgents control.

Religious and community leaders assembled crowds to walk from Sadr City, the Baghdad slum named after the cleric's slain father, to Imam Khadhimain mosque. The crowd, turning out on a Shiite holiday marking the death of a 9th-century imam, was large enough that thousands of worshipers laid carpets across the asphalt on the streets outside and prayed there.

The Reuters news agency reported that violence broke out as people were leaving the mosque. Iraqi police fired into the crowd, killing two people and injuring five; Sadr aides said the people were not armed.

The violence occurred as uncertainty mounted about Sadr's location and intentions. He and militiamen from his Mahdi Army abandoned a shrine in the city of Najaf under a negotiated settlement to end three weeks of fighting against U.S. and Iraqi forces late last month. Officials of Iraq's interim government have been trying to persuade Sadr to disarm the militia and turn it into a political organization, but no deal has been reached.

On Friday, men loyal to the cleric kidnapped four Iraqi policemen in Najaf and threatened to kill them unless Iraqi security forces stopped pursuing Sadr and his supporters, according to a video aired on the Arab television network al-Jazeera. The kidnapping came a day after Najaf police raided and searched Sadr's office.

Sadr had accepted a cease-fire under the Najaf agreement. With exceptions such as Friday's kidnapping, the Mahdi Army has been largely quiet in Najaf, but it has continued to fight U.S. forces in Sadr City.

On Friday, the thousands of people gathered at the Baghdad mosque heard Hazim Araji, a local sheik and an aide to Sadr, threaten more bloodshed if U.S. and Iraqi forces continued what Araji said were violations of the Najaf truce.

Araji mentioned attacks this week in Fallujah, 35 miles west of Baghdad, and Tall Afar, a city near the Syrian border that U.S. forces have bombed and encircled in an effort to drive out what the military has described as a "large terrorist element."

One Iraqi man was killed in bombing Friday in Fallujah, according to the Associated Press, quoting a physician there. The U.S. military said it has been trying to kill supporters of Abu Musab Zarqawi, a Jordanian linked to al Qaeda who is thought to be using Fallujah as a base of operations. But Arab television has shown repeated images of civilians, including young children, who it reported were killed or injured in the strikes.

In Tall Afar, an estimated 67 insurgents have been killed since the operation began Thursday, according to Lt. Col. Paul Hastings, a spokesman for the Army's 2nd Infantry Division, which has taken part in the fighting. There have been no American casualties, he said.

Hastings said most of the casualties resulted from three airstrikes against insurgents who had driven out the local U.S.-installed government and launched attacks against U.S. and Iraqi forces in recent weeks. Hastings said the U.S. military intervened after receiving a request from the provincial government in Ninevah.

Hastings said U.S. commanders believe 250 to 300 insurgents remain in Tall Afar, including some foreigners who may have come there from Fallujah. He said the insurgents appeared to have rocket-propelled grenades, small arms and mortars.

Hastings said that the insurgents had left local government ineffective and that the operation would not end until it had been reinstalled.

U.S. forces had set up checkpoints to monitor movements in and out of the city, Hastings said. In addition, the Iraqi Red Crescent was assisting hundreds of people who have been displaced by the fighting.

"This isn't over," he said. "The people of Tall Afar want a legitimate government."

Special correspondent Khalid Saffar contributed to this report.

Iraqi Shiites attend Friday prayers at Imam Khadhimain mosque in Baghdad to support Moqtada Sadr. Meanwhile, the U.S. kept up attacks on two cities.